In the particular case of tubes, what takes place during sizing is essentially a reduction in the diameter of a semi-finished product, in order to reach its nominal value, because the thickness of the wall is usually reached through previous stages of the production cycle which can be carried out by rolling the material around a mandrel using rolling mills of a type commonly known by the abbreviation MPM, or on so-called push benches, or on machines known as "plug mills" etc. See for example on the matter the Japanese Patent Application No. 07314013.
Consequently the sizing of a tube may include the stage of withdrawing the mandrel, if this is made necessary by the previously executed process; that of reduction, whether by stretching or otherwise, and that of finishing with the aim of finally reaching the tolerances required in the production cycle.
Similar considerations to those indicated above with regard to tubes can also be applied, with the appropriate modifications, to the cylindrical bodies referred to earlier.
At present, the usual preference is for sizing to be carried out on rolling mills employing rolling units or stands (both of these terms will be used hereinafter with the same meaning) containing three driven rolls, as these have been found to give good performance as regards the surface finish of the wrought products.
With this type of rolling mill, a very common configuration is that in which the three rolls of the rolling stands are arranged with their respective axes of rotation lying along the sides of an equilateral triangle and are connected to each other via bevel gears, enabling them to be rotated simultaneously by the application of drive to only one of them; in this form the adjustment of the distance between the rolls and the rolling axis is small because the abovementioned configuration will not allow any large movement, it being necessary to keep the bevel gears in mesh. In practice, with this type of rolling mill it is possible to make only a very small adjustment in the positions of the rolls, by adjusting the supporting mechanisms of their bearings; most significantly, it should be pointed out that, under load, that is to say during the processing of a tube or cylindrical body, there is no possibility of making any adjustment.
Because of the low operational flexibility of rolling stands of this type, for brevity's sake they will be referred to below as fixed or fixed-type stands.
It should also be noted that the low flexibility of rolling mills made up of this type of stand requires that for output ranges of tubes comprising a large number of diameters, a large bank of rolling stands must be prepared, because in practice each diameter of tube to be made requires the availability of a set of stands set up specifically to form the rolling mill.
In order to remedy this situation, special rolling units with three or more driven rolls whose positions with respect to a rolling axis can be adjusted even under load, have recently been developed; examples of such rolling units are disclosed in the Italian Patent Applications Nos. MI92A000917 (also European Patent Application No. 92118389.3) and MI93A000704 (also International Patent Application No. PCT/EP/93/00898), both laid open and filed by the owner of this application. In these rolling units the rolls are mounted at the ends of respective lever-type arms which are hinged, at the opposite end from the rolls, to a container having the form essentially of a closed frame that can be removed from an external supporting structure of the rolling unit: each roll is also provided with a mechanism for adjusting the distance between it and the rolling axis, the mechanism acting on the metal blocks that support the bearings of the rolls.
Also known are rolling mills in which the positions of the rolls of each stand are adjustable with respect to the rolling axis by means of a cam system.
In order to distinguish them from the fixed stands referred to earlier, or brevity's sake this second type of rolling unit will here be referred to as an adjustable or adjustable-type unit.
The prevailing trend at the moment in rolling mill manufacture is for an all-identical configuration: this means that they use rolling stands or units that are all identical to each other; this is partly in order to achieve an economy of scale in the production of such stands or units, and partly because with this approach they are made interchangeable in a way that is advantageous when rolls are being changed: as is well known, operating a rolling mill involves periodic re-turning of the rolls as a consequence of the inevitable problems of wear which in the long term cause damage to their surfaces. The operation of machine-turning the rolls of the so-called finishing stands, that is the last stands in the series which produce the accurate diameter, is much more frequent than that of the roughing stands, because wear in the former cannot be tolerated beyond certain limits without compromising the final dimensional tolerance. When the thickness removed from each roll by the periodical re-turning becomes such that the profile of its groove is no longer suitable for producing a certain diameter of a tube or cylindrical body, the entire stand, or the roll container, in the case of rolling units as in the abovementioned patent applications, is moved one or more positions back up the mill to a position where the rolls can operate on a larger diameter.
However, in these rolling mills having all-identical configuration, difficulties arise in their management from a production point of view.
As far as fixed-stand rolling mills are concerned, the stands must be set up at the beginning of the working cycle and, once adjusted, their position can no longer be changed during the sizing of a piece because of their rigid manner of operation.
However, when it comes to mills whose rolling units can be adjusted even under load, the variations that are always present in the production parameters such as, for example, the temperature of a semi-finished product, the environmental conditions in which sizing is being carried out, the composition of the material of the semi-finished product and hence its suitability to being rolled, the possible presence of a reheating furnace between the first stands or units that carry out the extraction and/or first reduction of a tube and the final stands or units that carry out finishing, make the entire production control apparatus exceedingly complicated; it is scarcely necessary to point out that computerized control systems are used for this purpose which, in response to changes in the abovementioned parameters, send signals to the various rolling units to update the positions of the rolls on the basis of complex calculations.
Furthermore, although their performance is better than that of fixed-stand rolling mills, in the case of adjustable-type rolling units maintenance is required of all the control and adjustment systems. This inevitably raises the running costs when compared with the more conventional configuration of fixed-stand rolling mills; although, therefore, such an increase in costs may be justified in some cases, e.g. where great precision is required, in the case of work that is less demanding from this point of view it may cause a loss of competitiveness with regard to the price of the final product.
The object of the present invention is to provide a rolling mill for sizing tubes or cylindrical bodies in general in the iron and steel industry, having structural and functional characteristics that will make it possible to overcome the drawbacks described above.